1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method used for copiers, electrostatic printings, printers and electrostatic recordings.
2. Description of the Related Art
Various electrophotographic methods have been known and in general, a surface of a latent electrostatic image bearing member is charged and the charged surface of the latent electrostatic image bearing member is exposed to form a latent electrostatic image. And the latent electrostatic image is developed using a toner to form a toner image on the latent electrostatic image bearing member. Moreover, the toner image on the latent electrostatic image bearing member is transferred to a recording medium directly or through an intermediate transfer member and a record on which an image is formed on the recording medium is obtained by fixing the transferred toner image by application of heat, pressure or both at the same time. Meanwhile, the residual toner on the latent electrostatic image bearing member after toner image transferring is cleaned by known methods using blade, brush, roller, and the like.
Better image quality, digitalization, colorization and speeding-up are demanded as a trend of electrophotographic technologies nowadays. For example, high resolution of 1,200 dpi or more is being investigated and image forming methods of high resolution which is previously unheard of are demanded in order to realize the high resolution of 1,200 dpi or more. More reduction in particle diameters of toner and developer for visualizing the latent electrostatic images are also examined in order to form high-resolution images and it is in the process of being realized.
Moreover, uniformity of dots which form the images is required for corresponding to digitalization of the images and uniformity of toner which forms the dots is also required. For this reason, a pulverization toner which is granulated by thermal current granulation and current granulation or a spherical toner such as polymerized toner produced by suspension polymerization, emulsion polymerization and dispersion polymerization are advantageous over commonly used pulverized toners having nonuniform profile produced by mechanical pulverization.
Furthermore, a tandem type electrophotograhic technique, in which each toner image formed on the latent electrostatic image bearing member respectively is transferred to a recording medium directly or through an intermediate transfer member using a number of latent electrostatic image bearing members and developing units, is employed for speeding up of color image output. However, in order to correspond to further speeding up of image output, the roller used for developing must be rotated at a high velocity to substantially increase the developed amount per unit of time.
In this case, a layer thickness of toner on the developing roller is regulated by control member in single component development and the toner on the developing roller is mechanically stressed from stirring with carriers or height regulation of magnetic brush, etc. in two-component development, and the stress received by the toner is further increased by high rotation frequency of the developing roller. Consequently, the toner surface is coated with fine particles (external additives) for ensuring flowability, however, the mechanical stress has an impact on the toner and causes burial and separation, etc. of external additives. By this burial and separation of external additives, adhesion between toner and other members and between toners are increased and problems such as degradation of transfer ratio and increased probability of “hollow defects” occurrence, a defect in which a part of an image, especially the center part of thin lines is not transferred, arises. For this reason, measures such as reducing the mechanical stress, etc. are employed to prevent burial or separation of external additives, however, sufficient effect has not been obtained.
It is proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 2001-83735 and 2002-311638 that by using external additives of large particle diameters, which are unlikely to be buried in the toner, the increase in toner adhesion due to mechanical stress can be prevented.
Furthermore, “hollow defects” phenomenon during transferring is thought to occur because adhesion of the toner compressed with a pressure between an image bearing member and a transfer member, particularly the non-electrostatic adhesion independent from the charge of the toner increases, making the toner transfer uncontrollable by Coulomb's force of electrical field. For example, intensity range of non-electrostatic adhesion is defined in JP-A No. 2000-66441. Moreover, in JP-A Nos. 2001-318485 and 2001-255677, the range of dependence property of non-electrostatic adhesion on the toner particle diameter is defined to prevent “hollow defects”.
However, non-electrostatic adhesion is dependent on the particle diameter of external additives and the relation between non-electrostatic adhesion and particle diameter of external additives is not defined at all in the prior art documents and the relation between non-electrostatic adhesion when external additives of large particle diameters are used, and “hollow defects” is not being investigated satisfactorily.
Therefore, prompt provision of the image forming apparatus and the image forming method, which are capable of obtaining appropriate images stably even after long-term use with high transfer efficiency and no image defects such as “hollow defects”, is desired under the current circumstances.